Significance of proposed research: Untreated hypertension and renal injury are risk factors for increased morbidity and mortality in sickle cell disease, yet early markers of progressive disease have not been identified and therapies to prevent the development of adverse cardiovascular outcomes have not been defined. Circadian blood pressure, as defined by 24 hour blood pressure monitoring, is more accurate than clinic blood pressure in defining secondary hypertension and abnormal nocturnal blood pressured dipping has been linked to progressive renal disease in other diseases. Methodology/Aims: A randomized feasibility trial of losartan will be conducted among 40 adolescent HbSS and SB0 thalassemia patients (11-19 years) with abnormal nocturnal blood pressure dipping. During this six month feasibility trial, two dosing strategies of losartan (titraed to keep BP <95th percentile vs. <75th percentile) will be analyzed for safety and effect on restoring normal circadian blood pressure. A prospective cohort study among HbSS and SB0 thalassemia patients (6-19 years) will also be conducted to evaluate the incidence of hypertension and role of monitoring potential biomarkers of kidney injury and hypertension. Cohort participants will undergo annual evaluations of hypertension and markers of kidney injury (24 hour blood pressure monitoring for participants = 11yrs; Blood: uric acid; Urine: Cystatin-c, Kim-1, NGAL, B2M). Expected Results: At the completion of the feasibility trial, vital background information will be obtained to design a definitive multicenter trial of hypertension in sickle cel disease. At the completion of the cohort study, the incidence of pediatric hypertension will be identified and the role for monitoring blood and urine biomarkers will be better understood. As therapy for patients with renal failure is dismal, it is imperative that SCD patients at risk ar identified early and that therapeutic trials are conducted that prevent progression.